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Repair a low-probability race condition identified by Qingqing Zhou.

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If a process abandons a wait in LockBufferForCleanup (in practice,
only happens if someone cancels a VACUUM) just before someone else
sends it a signal indicating the buffer is available, it was possible
for the wakeup to remain in the process' semaphore, causing misbehavior
next time the process waited for an lmgr lock.  Rather than try to
prevent the race condition directly, it seems best to make the lock
manager robust against leftover wakeups, by having it repeat waiting
on the semaphore if the lock has not actually been granted or denied
yet.
This commit is contained in:
Tom Lane
2006-04-14 03:38:56 +00:00
parent cc39aca7d4
commit 0fcc3c2f1d
5 changed files with 44 additions and 53 deletions
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14
src/backend/storage/lmgr/lock.c
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@@ -8,7 +8,7 @@
*
*
* IDENTIFICATION
* $PostgreSQL: pgsql/src/backend/storage/lmgr/lock.c,v 1.163 2006/03/05 15:58:38 momjian Exp $
* $PostgreSQL: pgsql/src/backend/storage/lmgr/lock.c,v 1.164 2006/04/14 03:38:55 tgl Exp $
*
* NOTES
* A lock table is a shared memory hash table. When
@@ -1116,10 +1116,12 @@ WaitOnLock(LOCALLOCK *locallock, ResourceOwner owner)
}
/*
* Remove a proc from the wait-queue it is on
* (caller must know it is on one).
* Remove a proc from the wait-queue it is on (caller must know it is on one).
* This is only used when the proc has failed to get the lock, so we set its
* waitStatus to STATUS_ERROR.
*
* Appropriate partition lock must be held by caller.
* Appropriate partition lock must be held by caller. Also, caller is
* responsible for signaling the proc if needed.
*
* NB: this does not clean up any locallock object that may exist for the lock.
*/
@@ -1132,6 +1134,7 @@ RemoveFromWaitQueue(PGPROC *proc, int partition)
LOCKMETHODID lockmethodid = LOCK_LOCKMETHOD(*waitLock);
/* Make sure proc is waiting */
Assert(proc->waitStatus == STATUS_WAITING);
Assert(proc->links.next != INVALID_OFFSET);
Assert(waitLock);
Assert(waitLock->waitProcs.size > 0);
@@ -1151,9 +1154,10 @@ RemoveFromWaitQueue(PGPROC *proc, int partition)
if (waitLock->granted[lockmode] == waitLock->requested[lockmode])
waitLock->waitMask &= LOCKBIT_OFF(lockmode);
/* Clean up the proc's own state */
/* Clean up the proc's own state, and pass it the ok/fail signal */
proc->waitLock = NULL;
proc->waitProcLock = NULL;
proc->waitStatus = STATUS_ERROR;
/*
* Delete the proclock immediately if it represents no already-held locks.